1. Technical Field
The present invention relates to determining OVSF codes. More particularly, the present invention relates to generating OVSF codes by an integrated circuit (IC).
2. Related Art
Orthogonal Variable Spreading Factor (OVSF) codes are used in Code Domain Multiple Access (CDMA) communication systems. For example, the Uniform Mobile Telecommunication System (UMTS) used by Third Generation “3G” mobile (3G/UMTS) uses OVSF to offer mobile telephone operators significant capacity and broadband capabilities to support a greater number of voice and data customers—especially in urban areas. In the 3G/UMTS system, multiple channels are combined and transmitted simultaneously.
In order to be able to recover simultaneously transmitted data in 3G/UMTS, the transmitted symbols are “spread” using the OVSF code. The greater the Spreading Factor (SF) the lower the available symbol transmission rate, but the easier it becomes to recover the data due to the correlation gain at the receiver. The spreading factor will vary on a channel-by-channel basis. For UMTS, the SF can vary between 4 and 512.
For a typical UMTS system, around 128 transmission channels are used but a lot more channels may be available. An example transmission of a single channel is shown in FIG. 1. The symbols “1” and “−1” for transmission are initially shown in FIG. 1. Since multiple channels are typically supported simultaneously by a CDMA communication system, in order to generate the required transmission sequence, multiple OVSF codes are used to carry the symbols from multiple channels. As shown in FIG. 1, one OVSF code (code=5) having an SF=8 is mixed with the symbols from a user to generate the transmitted data sequence for one channel. The OVSF codes generated are mixed with user data as illustrated in FIG. 1 to preserve orthogonality between channels enabling differentiation between a user's different physical channels. In a typical worst case scenario a maximum SF is 512, so 512 OVSF codes of SF=512 are required to mix with a user's channels. If these codes were all stored in memory, 512×512 bits of storage is needed (256 K bits storage).
In order to reduce the memory required by a user in such a UMTS system, it would be desirable to provide minimal circuitry to generate the OVSF codes.